Laser marking of plastics has been developed in the last few years in order to apply markings which are, for example, individual for the marked item, such as a serial number on an appliance or a photograph on an identity card, or else resistant to scratches or solvents. Markings in high quality are generally accomplished with pulsed laser units with the wavelengths 1064, 532 or 355 nm (Nd:YAG, Nd:YO4 or FAYb), the focused laser beam being conducted over the polymer surface by means of swiveling mirrors. Some inscription tasks can, though, also be solved with pulsed or “continuous wave” CO2 lasers in the wavelength range from 9.3 to 10.6 μm.
According to the polymer and formulation, it is possible to apply markings in various ways:                gravure—low-contrast but permanent marking        ablation—removal of a (possibly colored) top layer and uncovering of a different-colored underlayer; good contrasts, but no color or gray shades possible        foaming—low contrast on light background, good contrasts possible with a dark background, gray shades not possible        color change—color change in the material caused by chemical reactions, for example carbonization of the plastic caused by local heating. Variation of the beam power enables gray or color shades.        
EP-A-0 866 750 describes laser-markable films for labels based on a white PET film which bears a black coating. Laser irradiation ablates the black coating and uncovers the white background. This structure enables good high-contrast white-on-black inscriptions and drawings. Since it is an ablation process, gray shades are not possible. In order to achieve a black-on-white inscription with this structure, the entire white surface would have to be scanned off with the laser, which is uneconomic and requires long periods in the course of irradiation by means of laser.
WO 03/18700 and U.S. Pat. No. 6,723,259 describe laser-markable labels which comprises a white layer and of a black layer. When the upper layer is white in color, a black-on-white inscription is possible with a short laser beam time. Owing to the ablation process, however, no gray shades are possible here either. The material used for the underlayer (electron beam-curable polyurethane acrylate) is brittle and has a low mechanical strength.
To achieve high-contrast markings with the possibility of gray shades (for example for photographs), the route through color change (carbonization) is required.
Compared to the few studies described above in the field of laser-markable coated films, there is a multitude of publications regarding laser-markable polymer molding materials. However, the patent literature and other literature contains contradictory statements regarding the necessity of specific “laser additives” for one plastic or another. This is presumably because particular additives which are regularly added to plastics for other purposes (for example as a filler, for coloring or for flame retardation) can also promote the laser marking result. The literature particularly frequently mentions polycarbonate, PBT and ABS as “laser-markable even without additive”, but additives are often added even in the case of these polymers in order to improve the laser markability further.
In order to promote and to support the color change in polymer molding materials, various additives have been developed. As a result of the addition of an “absorber”, a substance which absorbs the laser light and converts it to heat, the heat input and the carbonization can be improved. This is the case even for polymers such as polycarbonate which carbonize readily of their own accord. EP-A-0 198 771 describes the use of antimony metal or antimony oxide as an absorber. In JP-A-02-226470, carbonization is promoted by the absorber carbon black. In DE-A-199 61 304, coated graphite is used as an absorber for laser marking. DE-A-195 22 397 shows the use of mica (sheet silicate) coated with metal oxides as an absorber. Absorbers based on tin-antimony mixed oxides are described in U.S. Pat. No. 6,693,657. In WO 2006-42714, the dark coloration of plastics is obtained by the use of additives based on various phosphorus-containing mixed oxides of iron, copper, tin and/or antimony.
When a color change by carbonization is desired in a plastic which can be carbonized only moderately, if at all, an absorber can be combined with a readily carbonizing material, as proposed, for example, in EP-A-0 991 523. WO 2004/50766 and WO 2004/50767 disclose a laser marking additive with a particular structure, in which the absorber is present fixed in a core-shell structure with the readily carbonizing material.
Polyvinyl chloride films (PVC films) are frequently used in the production of cards, for example credit cards, which have to be individualized. Often, the cards are accordingly embossed with numbers and text, i.e. deformed thermoplastically, but this process is not suitable for every type of personalization. The processibility of the PVC films to cards is generally referred to as good provided that no laser marking is required. The thermally embossed individualization features can, however, be damaged in hot environments, for example in the interior of an automobile in the sun. There is therefore an interest in laser marking. However, laser marking of PVC films is problematic. The reason for this is that PVC decomposes at the high temperatures of several hundred degrees which occur in the course of laser irradiation in a zip reaction with release of gaseous, corrosive hydrochloric acid. The release of gaseous hydrochloric acid necessitates specific, particularly acid-resistant suction and filter apparatus for the laser marking, which are supplied, for example, by Bofa (Dorset, UK). Owing to the gas evolution and the associated material ablation, PVC is suitable only for gravure markings. In order to obtain good contrast, a printed-on colored lacquer layer is also ablated in the gravure process.
In addition to the difficulties in the laser marking, PVC cards break relatively frequently and often have to be replaced after a short time. A more robust laser-markable film material which does not release a corrosive gas in the course of laser processing would be advantageous.
Polycarbonate cards are much more robust than PVC cards and can be laser-marked very efficiently even with good gray shades owing to the low carbonizability, for example for the reproduction of photographs in identity cards and driving licenses. A disadvantage in the case of polycarbonate is the high material costs. Moreover, polycarbonate is not very stiff, since the polymer has a relatively low modulus of elasticity, which is manifested in a relatively low scratch resistance and relatively poor processability in card production.
Nor has there been any lack of attempts to find laser-markable alternative materials.
DE-A-196 31 283 and EP-A-1 458 574 describe laser-markable laminated cards which have core layers of ABS and outer layers of amorphous polyesters (PETG, PCTG) or acrylate. However, the use of amorphous, unoriented polyesters in the outer layer means that such cards are not very robust.
JP-A-07-276 575 and JP-A-2002-273 832 disclose laser-markable three-layer cards comprised of a PBT core layer and two PET outer layers by lamination by means of an adhesive or coextrusion. In both cases, the PBT layer comprises laser absorbers. Since at least the PBT layer and probably even all layers are unoriented, the stiffness is low. In the course of laser marking, the PET layer crystallizes, as a result of which the contrast is not optimal.
White-colored mono- or multilayer, biaxially oriented, heat-set PET films are known (see, for example, in EP-A-1 256 597 or EP-A-1 125 967) and are less expensive than polycarbonate films. Although these films have the desired stiffness and strength and are very suitable for the production of labels or cards, they are not sufficiently laser-markable. EP-A-0 605 130 describes coextruded polyester films which comprise a white-colored outer layer and a transparent outer layer. These films too have the desired stability and strength, but are not sufficiently laser-markable either.